DESCRIPTION: The otolith organs of the vestibular system transduce linear acceleration and tilt with respect to gravity. Sensing of tilt and linear translation is ambiguous, since both involve variations in the resultant gravito- inertial force. Some studies indicate that otolith information is partially separated into high and low-frequency pathways, subserving translati and tilt respectively, for the generation of compensatory eye movements. Perceptual mechanisms are presumably subject to the same tilt-translation ambiguity, and the investigator hypothesizes that frequency filtering is used resolve this perceptual ambiguity also. In particular, the hypothesis is that low-frequency otolith signals indicate tilt; however, several studies have shown that subjects perceive (correctly) translational motion even at low frequencies where a tilt response would be expected. The investigator suggests that this may be due to non-vestibular cu (noise and vibration) that are present during the experiments and that provide information about translation. A set of experiments will characterize the dynamics of the perception of translational motion. Experiments are also proposed which will assess and dissociate vestibular and non-vestibular cues in several ways: 1) by using two sleds (one, the NASA sled at Ames Research Center, has extremely low noise and vibration); 2) by using the Rochester sled in a way that generates noise and vibration cues that are not consistent with normal experience while translatin and 3) by using patients without vestibular function. Finally, a set of experiments is planned in which eye movements will be measured while subjects are translated.